Mass flow meters for flowing media that work on the Coriolis Principle are known in various embodiments (see, for example, German Disclosure Documents 26 29 833, 28 22 087, 28 33 037, 29 38 498, 30 07 361, 33 29 544, 34 43 234, 35 03 841, 35 05 166, 35 26 297, 36 32 800, 37 07 777, 39 16 285, 40 16 907, 41 24 295, 41 24 296 and 41 29 181, European Patent Disclosure Documents 0 083 144, 0 109 218, 0 119 638, 0 196 150, 0 210 308, 0 212 782, 0 235 274, 0 239 679, 0 243 468, 0 244 692, 0 261 435, 0 271 605, 0 275 367 and 0 282 552, as well as U.S. Pat. Nos. 4,491,009, 4,628,744, and 4,660,421) and are increasingly being used in practice.
Mass flow meters for flowing media that work on the Coriolis Principle are basically divided into those whose measuring pipes are designed to be at least basically straight, and those whose measuring pipes are designed to be loop-shaped. The mass flow meters in question are also divided into those with only one measuring pipe and those with two; in designs with two measuring pipes, the pipes may be fluidically in series or in parallel.
Embodiments of mass flow meters in which the measuring pipe or pipes are designed to be straight are simple in mechanical design and consequently can be produced at relatively low cost. Moreover, the inner surfaces of the pipe are easy to work on, for example, to polish and they have low pressure losses.
The disadvantage of mass flow meters that work on the Coriolis Principle and in which the measuring pipe or pipes is designed to be straight is that both thermally caused changes in length and thermally caused stresses and also forces and torques working from those outside can lead to measurement errors and to mechanical damage, namely to stress cracks.
Experts have already dealt with the measurement errors that occur due to temperature changes in mass flow meters that work on the Coriolis Principle.
First of all, it has already been recognized that the temperature dependence of the modulus of elasticity influences the oscillation frequency and the elasticity of the measuring pipe and thus the measured value; the result is that a temperature sensor is often provided that detects the temperature of the measuring pipe to correct the measured value depending on the temperature of the measuring pipe; see, for example, in the German publication "Messen Prufen Automatisieren", 1987, Vol. 23, No. 5, Pages 301 through 305, the essay Direkte Massendurchflussmessung, insbesondere mit Coriolisverfahren" by von W. Steffen und Dr. W. Stumm.
Incidentally, with a mass flow meter of the type described at the outset, the extensive temperature dependence of the measured value was taken into account in that a temperature sensor is provided to detect the temperature of the carrier pipe to correct the measured value, depending on the temperature of the carrier pipe (see German Disclosure Document 36 32 800 and the corresponding European Disclosure Document 0 261 435). Here, temperature sensor signals produced by the two temperature sensors (one for the measuring pipe and one for the carrier pipe) are put into a correction circuit that should eliminate the influence of temperature on the measured value. Specifically, provision is made for the correction circuit to multiply the measured value by a correction factor K=K.sub.0 +K.sub.1 T.sub.1 +K.sub.2 T.sub.2 +K.sub.3 T.sub.1.sup.2 +K.sub.4 T.sub.2.sup.2 +K.sub.5 T.sub.1 T.sub.2, wherein T.sub.1 is the temperature of the measuring pipe, T.sub.2 is the temperature of the carrier pipe and K.sub.0, K.sub.1, K.sub.2 K.sub.3, K.sub.4 and K.sub.5 are constant coefficients that are specific for a certain embodiment of the mass flow meter.
In practice, it was shown that the higher order terms of the above expression can be ignored, so that temperature compensation is attained with sufficient precision if the uncorrected measured value is multiplied by the correction factor K=K.sub.0 +K.sub.1 T.sub.1 +K.sub.2 T.sub.2. In the known mass flow meter described above, the temperatures of the measuring pipe and the carrier pipe--more or less as the external cause of a temperature-dependent measurement error--are considered correcting; the internal causes resulting from these external causes have not yet been addressed, however.
Finally, a mass flow meter working on the Coriolis Principle is known that, like the mass flow meter from which the invention comes, has a straight measuring pipe carrying the flowing medium, an oscillator acting on the measuring pipe, two measurement transducers that detect Coriolis oscillations based on Coriolis forces and a carrier pipe for the measuring pipe, the oscillator and the transducers, but in which there is no temperature sensor to detect the temperature of the measuring pipe, but where, in another way, care is taken that the measured value is largely non-temperature-dependent, and temperature changes thus do not lead to measurement errors to a considerable extent (see German Disclosure Document 41 24 295 and corresponding U.S. application Ser. No. 07/917,577, infra). In this mass flow meter, the carrier pipe is designed as a so-called compensation cylinder, through which or in connection with which temperature changes--as well as forces and torques acting from the outside--are compensated or at least their effects are largely eliminated. The structural unit of the measuring pipe and the carrier pipe designed as a compensation cylinder is more or less "immune" to temperature changes, and to forces and torques acting from the outside.
Thus, additional measures to "immunize" the cylinder from temperature changes and forces and torques acting from the outside are taken. An initial additional measure of this kind consists of the fact that the measuring pipe is arranged inside the carrier pipe with tensile prestress. As the temperature increases, the tensile prestress decreases. A second extra measure to "immunize" the cylinder consists of using materials for the measuring pipe and the carrier pipe with the same or almost the same heat expansion coefficients, especially materials with relatively low heat expansion coefficients. For further details on this known mass flow meter, please refer expressly to the contents of German Disclosure Document 41 24 295 and corresponding U.S. application Ser. No. 07/917,577, filed Jul. 21, 1992, the contents of which are hereby incorporated by reference herein.